1. Field of the Invention
The present invention relates to the production of phosphoric acid and more particularly refers to an improved wet process for converting phosphate rock into a concentrated phosphoric acid with concomitant production of an improved calcium sulfate.
In conventional wet methods of producing phosphoric acid, finely divided phosphate rock is digested with a mixture of phosphoric acid, sulfuric acid and water causing a reaction between the calcium phosphate in the rock and the sulfuric acid to produce phosphoric acid and a precipitate of calcium sulfate (either the anhydrite form--CaSO.sub.4 with no water of hydration; the hemihydrate form--CaSO.sub.4. 1/2 H.sub.2 O; or the dihydrate form--CaSO.sub.4.2H.sub.2 O). Over the years numerous improvements and refinements in these methods have been proposed, many of which are described in Phosphoric Acid edited by A. V. Slack (published by Marcel Dekker, Inc., New York, 1968). Many of these refinements are directed to producing the calcium sulfate co-product in various forms.
Most wet processes are directed to producing the calcium sulfate product in the dihydrate form, or phosphogypsum. In most commercial operations, the plants are run at about 75.degree.-80.degree. C. and 30% P.sub.2 O.sub.5 in the leach zone. Here the reaction of sulfuric acid and apatite (phosphate rock) is over in about 15 minutes. However, a residence time of about 4 hours is needed for gypsum crystal growth to sufficiently large crystals for filtration. Generally, the practical maximum P.sub.2 O.sub.5 concentration of the phosphoric acid for a dihydrate process is around 29-30% (40-41% H.sub.3 PO.sub.4), which must subsequently be concentrated to higher levels, while the operational temperature is about 75.degree.-80.degree. C. Above these temperatures and P.sub.2 O.sub.5 concentration limits, the phosphogypsum becomes unstable during operations so that increased amounts of the metastable hemihydrate are formed, with attendant hardening and setting up in mixing vessels and greatly increased reaction times. Further concentration of the phosphoric acid to more concentrated products requires energy intensive and expensive evaporators. A phosphoric acid of about 42% P.sub.2 O.sub.5 (60% H.sub.3 PO.sub.4) is needed because fertilizer manufacturers use this concentration to make diammonium phosphate.
A less commercially attractive variant on the wet process is the hemihydrate process. The hemihydrate processes may be useful in obtaining a calcium sulfate for building materials manufacture. However, the operator must be careful that the filters do not cool since the hemihydrate can set to gypsum. Then jackhammers are needed to remove it. Further, the hemihydrate product is objectionable because impurities such as radium in the ore and excess phosphoric acid are carried over into the hemihydrate. Here again, the phosphoric acid product, about 32% P.sub.2 O.sub.5, must be concentrated for fertilizer production.
According to data published in the Slack text volume 1 part 1, in an article on hemihydrate and anhydrite processes in Europe by P. M. R. Versteegh and J. T. Boontje, phosphoanhydrite can possibly be produced in a phosphoric acid attack system provided the temperature and concentration of the acids are sufficiently high, about 135.degree. C. and about 80% H.sub.3 PO.sub.4. For example, it is there proposed that anhydrite could be made at 95.degree.-100.degree. C. in a mixture of 42% P.sub.2 O.sub.5 phosphoric acid and 3-3.5% sulfuric acid; or at 85.degree.-90.degree. C. in the presence of 48-52% P.sub.2 O.sub.5 phosphoric acid. In the first case, a 75-82% sulfuric acid would be required; and in the second case, a 78% sulfuric acid would be required for the phosphate rock in the leach zone. The reference appears devoid of any mention of using anhydrite seed crystal in the proposed processes.
2. Description of the Prior Art
U.S. patent 2,531,977 discloses a dihydrate wet process for producing phosphoric acid with the suggestion that the phosphogypsum obtained be thereafter treated with 15-33% sulfuric acid at a temperature of from 60.degree. C. to the boiling point of the sulfuric acid solution for periods of more than 30 minutes (generally 2-3 hours) to obtain a calcium sulfate anhydrite. Nothing is known about the purity or practical usability of this anhydrite.